This invention relates to semiconductor processing and more particularly to chemical mechanical planarization (CMP) using polishing pad stiffness control with magnetically controllable fluid that causes a rheology change (a magnetorheological fluid).
In semiconductor device fabrication, silicon wafers, after undergoing a pattern, deposition, etching, etc. process to form electrical devices and circuits on a layer there is coated a layer of glass or oxide that is polished. In the case of adding another layer of devices and circuits an interconnect between layers is provided by forming a hole in the layer of glass or dielectric and filling the hole with copper and polishing off the copper. Chemical-mechanical polishing (CMP) is widely used as a process for achieving global planarization in semiconductor manufacturing. The pattern on the wafers below the top surface makes the polishing rate non-linear. The hills and valleys resulting from the products under the glass oxide make for the non-linear polishing. Differences in Chemical Mechanical Polishing (CMP) pressure and polish pad properties result in removal rate and thickness non-uniformities within-water (WIWNU). Areas of higher pressure or stiffer pad properties result in higher rates and thinner post-CMP thickness in certain areas.
Control of removal rate in specific areas on the wafer is currently very difficult, requiring multi-chamber pressure control to different parts of the wafer. Other solutions require set-up wafers to determine a multi-zone pressure profile for copper removal uniformity.
The prior art includes using varied stiffness sub-pads that provide the same properties throughout the entire pad and the properties are not variable. In another prior art, the pressure profile of the entire pad is changed with an inflatable bladder under the entire platen. It must be tuned with extensive set-up wafers and the properties are not variable across the wafer.
Another prior art method uses pressure-controlled CMP heads. The pressure chambers must be individually calibrated for a given ECD profile incoming to CMP, requiring extensive set-up wafers. There is no in-situ control.
In accordance with one embodiment of the present invention a method of polishing includes the steps of providing a pad containing magnetically controllable rheological (magnetorheological) fluid and controlling the stiffness of the pad by applying magnetic field at the magnetorheological fluid.
In accordance with another embodiment of the present invention an improved polishing pad for polishing an object includes a top porous polishing pad portion with asperity surface to aid slurry action and a sub-pad portion having pockets of magnetorheological fluid for causing stiffness control under a magnetic field.
In accordance with another embodiment of the present invention an improved CMP polishing apparatus is providing including a platen/table and a polishing pad on the platen/table; a polishing head for holding an object to be polished and providing pressure of the wafer against the polishing pad on the platen/table. A sub-pad between the polishing pad and the platen/table for providing mechanical support under the polishing pad. The sub-pad contains regions or pockets of magnetorheological fluid about the pad for controlling the stiffness in real time control. A magnetic field generator applies a selective magnetic field to selective regions or pockets of magnetorheological fluid in the sub-pad to control the stiffness of the sub-pad to provide real time control of the polishing of regions of the object being polished.